This invention relates to metal-modified phenol-aldehyde novolak resins, and more particularly to an improved method for producing them. Such metal-modified novolaks are useful as developing agents for colorless chromogenic dye precursors such as Crystal Violet lactone. Color producing systems of this type are useful in manifolding systems wherein a color-developer, a color precursor and a common solvent for each are maintained in isolation on a surface of a carrier such as paper, or in isolation on separate surfaces of a carrier. Conveniently, the color precursor and the solvent may be encapsulated in microcapsules of the type described in U.S. Pat. No. 2,800,457, and said microcapsules together with an adhesive applied as a coating to a web of paper. The color developer may be applied as a coating to a second web of paper, either alone or admixed with mineral particles. When the two webs of paper are superimposed with their respective coatings in contact and subjected to localized pressure as by a typewriter key, the walls of the microcapsules in the area subjected to pressure are ruptured, and the solution of color precursor and solvent originally contained in the ruptured capsules is transferred in a marking pattern to the paper surface carrying the color developer. Almost instantaneously a colored mark is formed on the surface carrying the color developer. Alternately, the microcapsules and color developer may be intermixed on the same surface of a paper web. If desired, the microcapsules may contain a solvent solution of the color developer, in which event the color precursor is applied as a coating to the second web of paper, or is intermixed with the microcapsules on the same paper surface.
The use of metal compounds as a means of improving color-forming reactions of phenol-aldehyde novolak resins with basic colorless chromogenic dye precursors is known. Acidic water soluble metal salts incorporated in aqueous coatings of ground novolak resins are taught in U.S. Pat. No. 3,516,845, while U.S. Pat. No. 3,723,156 teaches a further improvement through similar use of oil-soluble metal salts. U.S. Pat. No. 3,732,120 and 3,737,410 discloses the interaction of a metal compound such as zinc hydroxy benzoate, zinc acetylacetonate and zinc dibenzoate, with a para-substituted phenol-aldehyde novolak resin by melting the resin and the metal compound together. The melt is ground after cooling and then applied to a paper surface. It is stated that the metal-resin product gives increased color intensity and resistance to fading as compared to the novolak resin alone when used in color forming reaction with basic colorless chromogenic dye precursors such as Crystal Violet Lactone.